1. Field of the Invention
This invention relates to a superconducting coil apparatus, and more particularly, to a superconducting coil apparatus used in the linear motor of a magnetic floating train, the propelling motor of a electromagnetic propelling ship and the like.
2. Description of the Related Art
A racetrack type superconducting coil apparatus, as an example of a superconducting coil apparatus, is provided with windings formed from superconducting wires wound in a racetrack form and firmly fixed to each other by epoxy resin, and a cryostat containing the windings and so formed in a racetrack shape as to correspond to the shape of the windings. The outer and inner peripheral walls of the cryostat are disposed on the radially outer and inner of the windings. Liquid helium passages are defined between the outer and inner peripheral walls of the cryostat and the windings. When the windings are cooled lower than the transition temperature, their state is changed from the normal-conducting state to the superconducting state. Upon exciting the windings in the latter state, a high-intensity magnetic field is generated from the windings without any electrical loss.
While the windings are being excited, hoop stresses which are large magnetic forces are applied to the windings in the radial directions. If the windings are deformed so as to tend to assume a true circle form, it is likely that the superconducting wires are quenched and their state tends to be changed to the normal-conducting state. In order to avoid this, the windings are fixed to the outer and inner peripheral walls of the cryostat by means of a plurality of fixtures placed between the windings and the outer and inner peripheral walls of the cryostat (that is, in the liquid helium passages). The fixtures have holes for passing liquid helium. The opposed inner peripheral wall portions of the cryostat are connected to each other by means of reinforcing members. This structure prevents the most degree of deformation of the windings.
However, not so many fixtures can be arranged in the liquid helium passages because the arrangement of many fixtures lowers the cooling efficiency of the winding unit. This arrangement cannot fully hinder the windings from being deformed and thus it is possible that the windings change their shape slightly. The deformation of the windings causes rubbing between the fixtures and the windings to generate frictional heat. Although part of the frictional heat is absorbed by the liquid helium, the remainder of it is transferred to the superconducting wires in the outer and inner peripheral areas of the windings and quenches them. As a result, the superconducting wires of the overall windings are frequently quenched.